The electrostatographic process, and particularly the xerographic process, is known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development of the image with a developer, and subsequent transfer of the image to a suitable substrate. In xerography, the surface of an electrophotographic plate, drum belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive developer particles are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that enable continued formation of developed images of high quality and excellent resolution. In two component developer compositions, carrier particles are used in charging the toner particles.
Carrier particles in part comprise a roughly spherical core, often referred to as the “carrier core,” which may be made from a variety of materials, such as magnetic materials. The core is often coated with a resin. This resin may be made from a polymer or copolymer. The resin may have conductive material or charge enhancing additives incorporated into it to provide the carrier particles with more desirable and consistent triboelectric properties. The resin may be in the form of a powder, which may be used to coat the carrier particle. Often the powder or resin is referred to as the “carrier coating” or “coating.”
Known methods of incorporating conductive material into carrier coating include the use of electrostatic attraction, mechanical impaction, in-situ polymerization, dry-blending, thermal fusion and others. These methods of incorporating conductive material into carrier coatings often result in only minimal amounts of conductive material being incorporated into the coating or produces conductive carrier coatings too large for effective and efficient use in some of the smaller carriers. Other conductive coating resins use dry-blending processes and other mixing to incorporate the carbon black or other conductive material into the polymer. However, in order to avoid transfer of carbon black from conductive polymers so obtained, the amount of carbon black that can be blended is severely limited, e.g., to 10% by weight or less. This in turn severely limits the conductivity achievable by the resultant conductive polymer.
In addition to the problems associated with loading conductive materials into coating resins, recent efforts to advance carrier particle science have focused on the attainment of coatings for carrier particles to improve development quality and provide particles that can be recycled and that do not adversely affect the imaging member in any substantial manner. Many of the present commercial coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes causing failure upon impact or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate.
Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. High relative humidity hinders image density in the xerographic process, may cause background deposits, leads to developer instability, and may result in an overall degeneration of print quality. In the science of xerography, the term “A Zone” is used to refer to hot and humid conditions, while the term “C Zone” is used to refer to cold and dry conditions. Triboelectric charges are usually lower in the “A Zone” than in the “C Zone.” It is desirable to have the measured triboelectric charges (tc) for a particular carrier in the A Zone and the C Zone, when entered into a ratio of A zonetc/C zonetc, to be close to 1.0 in order to obtain good development in high humidity.
A carrier coating commonly used is #MP-116 PMMA available from Soken Chemical in Japan. This powder typically has a diameter of 0.4 to 0.5 micrometers and is a made from polymethyl methacrylate. However, it is required to use high amounts of #MP-116 PMMA to coat 30 to 150 micrometer carrier cores to achieve surface area coverage on the carrier of 85% to 95%. Use of such high amounts of carrier coating often results in lower carrier yields due to fused aggregates. Fused aggregates must be broken up or removed by screening. Crushing or breaking up of the aggregates may result in weak or “chipped off” areas on the carrier surface potentially causing poor coating quality. Screen separation may result in a lower yield as aggregates are removed from the final product.
Various coated carrier particles for use in electrostatographic developers are known in the art. Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example U.S. Pat. No. 3,590,000. These carrier particles may comprise various cores, including steel, with a coating thereover of fluoro-polymers and ter-polymers of styrene, methacrylate, and silane compounds.
There is a continuing need to be able to incorporate conductive material into coating resins while providing for and maintaining desirable xerographic qualities such as high coating efficiency, proper performance, and stable charging characteristics.